Function is_asset_drag 

pub(crate) fn is_asset_drag(payload: u64) -> bool

Whether payload carries the asset tag, regardless of how stale it is.

Kept separate from unpack_asset_drag because the two answer different questions: this one classifies the payload, the other resolves it. Routing classification through the epoch check would make a stale asset drag look like a packed EntityId and get handled as a reparent.